1. Field of the Invention
The present invention relates to a semiconductor laser, a semiconductor optical integrated device, an optical module and an optical communication system.
2. Description of the Related Art
A buried type semiconductor laser is widely used as a light source for optical communications in the wavelength band of 1.3 to 1.55 xcexcm (micrometer) because it can be highly efficiently coupled to a single-mode optical fiber and also it has an excellent oscillating characteristic. Particularly, from the viewpoint of reducing driving current, semiconductor lasers have been strongly required to perform a high slope efficiency operation.
In order to perform the high slope efficiency operation in the semiconductor lasers, it is necessary to reduce the leak current and the absorption loss. With respect to the leak current, a pnpn thyristor or high-resistance semiconductor is used as a current block layer to implement an excellent oscillation characteristic. Particularly, the current narrowing structure based on the pnpn thyristor is widely used because this structure can be relatively easily established by merely changing dopant and also it has a high withstand voltage characteristic. On the other hand, with respect to the absorption loss, a low-loss waveguide is implemented by introducing a distorted quantum well structure. That is, the band structure of the valence band is varied by introducing in-plane compressive distortion into the well layer having the quantum well structure, whereby absorption in the long wavelength band can be greatly reduced.
When these semiconductor lasers are manufactured, MOVPE (Metal Organic Vapor Phase Epitaxy) is most frequently used for crystal growth. This is because in-plane uniformity of crystal quality and reproducibility are excellent and a high-quality multiple quantum well (MQW) layer can be grown.
Owning to the crystal growth technique, the introduction of the quantum well/distorted quantum well structure into an active layer (active region) and a buried structure having an excellent current narrowing structure, the oscillation characteristics of the semiconductor lasers have been greatly improved. At present, a high-performance semiconductor laser having a threshold current of several mA (milliampere) is practically used.
However, in the process of manufacturing the semiconductor lasers, MOVPE growth is required to be carried out at plural times, and there is such a fear that high-concentration Si deposited on the re-growth interface affects the oscillation characteristic. The high-concentration Si is estimated to occur at the re-growth interface as follows. That is, Si components which exist in the air or contaminated into echant are deposited on the surface of the semiconductor and then taken into crystals at the re-growth time.
An estimated oscillating characteristic when high-concentration Si is doped into the re-growth interface will be described with reference to FIG. 1.
A normal buried type semiconductor laser is manufactured by carrying out the crystal growth operation at three times. Therefore, it has two re-growth interfaces, and Si concentration at these portions is increased. Re-growth interface 31 and re-growth interface 32 in FIG. 1 correspond to these portions. However, the re-growth interfaces which come into contact with an n-InP current block layer and an n-InP substrate little affect the oscillation characteristic and the electric characteristic because the same donor doping is carried out. At the re-growth interface 31 shown by a solid line of FIG. 1, particularly at the portion just above an active layer, a high-concentration donor doping layer is inserted into a p-layer to increase the resistance of this portion, so that the leak current is remarkably increased. The leak current serves as the base current of the current block layer to reduce the withstand voltage of the block layer, resulting in reduction in optical output. On the other hand, with respect to the re-growth interface 32 shown by a broken line of FIG. 1, there is no problem at the portion of the re-growth interface 32 which comes into contact with the n-InP substrate, however, the portions of the re-growth interface 32 which come into contact with the side wall of the active layer and the p-InP layer greatly affect the oscillation/electric characteristics. This is because if an Si pileup layer having low electric resistance exists along the side wall, leak current flows along the side wall and thus the slope efficiency is lowered.
As described above, the Si concentration of the re-growth interface and the oscillation characteristic of the semiconductor laser are in close relation with each other. Therefore, in order to enhance the oscillation characteristic, a layer whose conduction type has been inverted to n-type is required to be re-inverted to p-type by some method to remove the high-resistance layer and the leak current leaking path.
An object of the present invention is to provide a novel semiconductor laser and a method of manufacturing the semiconductor laser in which a portion whose conduction type has been inverted to n-type due to Si pileup at the re-growth interface thereof is re-inverted to p-type to enhance the oscillation characteristic. Another object of the present invention is to provide a cheap optical module and an optical communication system by using the semiconductor laser.
In order to attain the above objects, according to a first aspect of the present invention, there is provided a semiconductor laser comprising an active layer (active region) provided between two different conduction types of semiconductor layers and current block layers surrounding the active layer, wherein one of the semiconductor layers has a first growth layer and a second growth layer which is formed on the first growth layer by a re-growth process after a growth process for the first growth layer, and the doping concentration of the first growth layer in the range of at least 0.1 xcexcm in thickness from the interface between the first growth layer and the second growth layer is in the range from 1.5 times to 5 times of the doping concentration of the second growth layer. The active layer (active region) is a layer (region) which a luminous transition to contribute to the laser operation is generated.
According to a second aspect of the present invention, there is provided an optical module formed by using at least one semiconductor laser of the present invention.
According to a third aspect of the present invention, there is provided an optical communication system formed by using at least one semiconductor laser of the present invention.
According to the present invention, Si deposited at the re-growth interface is taken in crystals as donor dopant in the re-growth process. In the following description, the effect on the oscillation characteristic when donor dopant exists at the re-growth interface will be quantitatively reviewed.
A commercially available LD simulator was used to analyze the oscillation characteristic. In this LD simulator, a current continuous equation, a Poisson equation and a rate equation are solved self-consistently. FIG. 2 shows a mesh pattern used for the analysis. The mesh interval is made minute at the active layer having strong non-linearity and the peripheral portion of the pn junction in order to prevent the solution precision from being degraded. FIG. 3 shows the dependence of the internal differential quantum efficiency xcex7i on the re-growth interface concentration.
In this analysis, the oscillation characteristic was analyzed while the concentration of the re-growth interface 31 of FIG. 1 just above the active layer was varied from p-type to n-type. Further, since the thickness over which the concentration variation occurs at the re-growth interface is proved to be equal to about 0.1 xcexcm from the SIMS (Secondary Ion Mass Spectroscopy) analysis, the calculation was made by using this fact. In the FIG. 3, circles represent the measurement result of the internal differential quantum efficiency xcex7i calculated on the basis of the dependence of the slope efficiency on the length of a resonator, and the solid line represents the calculation result. Both the measurement and calculation results are very coincidence with each other. If the re-growth interface is p-type, the effect on the oscillation characteristic is little. On the other hand, if the re-growth interface is n-type, the internal differential quantum efficiency is lowered, so that the slope efficiency trends to be reduced. If an n-type layer is formed just above the active layer in the p clad layer, the resistance of the n-type layer portion is increased and thus injection of holes into the active layer is disturbed. The holes thus disturbed flows as base current into a p current block layer to thereby increase leak current passing through the current block layer, so that the oscillation characteristic is expected to have remarkable optical output saturation.
As a method of reducing the Si concentration at the re-growth interface, in the case of GaAs type semiconductor, a long-term standby state is kept while a large amount of AsH3 flows. A leading theory to explain this effect is that H+ radicals contained in AsH3 removes Si adhering to the surface. However, this method is unfavorable because In contained in crystals are removed in the re-growth standby state of InP containing no As and further InAsP having short wavelength composition is formed at the re-growth interface. There is such a report that the Si concentration at the re-growth interface can be reduced by high-temperature/long-time standby under flow of PH3 in the growth standby of InP (H. Ishikawa, et al., Journal of Applied Physics, Vol. 71, p.3898, 1992). However, according to this method, the standby temperature is set to a high value (700xc2x0 C.) and thus crystallinity at the re-growth interface may be lowered.
According to the present invention, the Si concentration at the re-growth interface can be effectively lowered to achieve a semiconductor laser having an excellent oscillation characteristic without passing through a special pre-growth standby step in the re-growth process which has been hitherto carried out. Specifically, high-concentration Zn is doped into a portion which has been inverted to n-type (this portion will be hereinafter referred to as xe2x80x9cinverted n-type portion (area)xe2x80x9d) due to high-concentration Si adhering to the re-growth interface, thereby inverting the conduction type of this portion from n-type to p-type (this p-type portion will be hereinafter referred to as xe2x80x9cinverted p-type portionxe2x80x9d). It has been found from the SIMS analysis that the thickness of the area whose conduction type is inverted to n-type in the neighborhood of the re-growth interface is not more than 0.1 xcexcm (0.1 xcexcm or less). Therefore, by doping high-concentration Zn into an area of 0.1 xcexcm (more suitably on manufacture process 0.2 xcexcm) in thickness (larger than the thickness of the above n-type area), the n-typed high-resistance layer is inverted to a p-type layer, thereby enabling holes to be effectively injected into the active layer, so that there can be achieved a semiconductor laser having an excellent characteristic which can oscillate with a high slope efficiency.